The ordinary screw-nut combination is very inefficient due to the fact that the sliding surfaces are subject to output loads which result in large friction loses. Various devices have been used to reduce the friction such as the ball-nut, roller-nut, etc. The present invention is related to a device that is intended to perform the same function but more simply, more efficiently, and at lower cost.
Broadly, some current devices include a threaded shaft unit and a nut unit. The nut unit commonly comprises an assembly having an outer case that houses anti-friction elements engaging the threaded shaft or screw and normally surrounds it for load distribution purposes. Devices of this type can broadly be found in U.S. patent classification class 74-424.8 and class 74-89.15. For example, the U.S. Pat. No. 3,003,362 to Martens uses a toroidal shaped bearing element in engagement with a normal screw, but, such combinations of toroidal shapes engaging normal screw threads do not permit large forces to be transmitted. Martens also uses pintles at the end of the shafts carrying the toroidal shape for force transmission to a cage. The diminutive and weak form of the pintles limit the forces that are transmittable from the screw through the cage to the housing plus high stress and friction concentration. And lastly he utilizes a freely rotatable ring that merely holds the bearing elements in rolling contact with the screw threads and has no load bearing function whatsoever.
The earlier U.S. Pat. No. 4,048,867, to Saari, the present inventor, included three basic members a screw, rollers and a ring forming the housing. Rotation of the screw caused a planetary motion of the rollers resulting in a linear translation of the ring. This simple device for converting torque to linear force had one draw back, namely, due to the planetary motion of the rollers, the nut did not move at the same rate in direct relationship with the lead of the screw but, rather, at a lesser rate. Moreover, repetition of position after a number of cycles of motion and load reversals was not assured. Where accuracey and repetitiveness of position was not desired the device was adequate, however, where it was desired to repeat a position the device did not function adequately. The present device overcomes these defects.
Additionally, the U.S. Pat. No. 2,488,356 to Anderson, and the two U.S. Pat. Nos. 3,965,761 and 4,033,194 to Stanley show anti-friction devices that make use of the end face of rollers and frusto-conical surfaces on the rollers and cages for transmission of axial and radial forces from the screw through the rollers to the housing. As will be appreciated such devices are inefficient and limited in their load bearing capabilities.